New middle and late Smithian ammonoid faunas from the Utah/Arizona border: New evidence for calibrating Early Triassic transgressive-regressive trends and paleobiogeographical signals in the western USA basin

Abstract

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Latest Smithian (Early Triassic) ammonoid assemblages in Utah (western USA basin) and their implications for regional biostratigraphy, biogeography and placement of the Smithian/Spathian boundary

“…They also highlighted that (i) late Smithian strata (Fig. 2: base of "Unit C") correspond to a mid-ramp setting with frequent storm deposits and thus include bioclastic accumulations with densely packed ammonoids, and (ii) the SSB is concomitant to the maximum transgression in this area (Lucas et al, 2007;Brayard et al, 2013Brayard et al, , 2020. Ammonoid specimens are abundant in the base of "Unit C" but sometimes fragmentary and slightly deformed.…”

“…They exhibit classical shalelimestone alternations of the Early Triassic Thaynes Group (sensu Lucas et al, 2007), which represent relatively shallow sedimentary deposits in an epicontinental sea. The maximum extent of this epicontinental sea within the western USA basin occurred during the Smithian-Spathian transition (Lucas et al, 2007;Brayard et al, 2013Brayard et al, , 2020Vennin et al, 2015;Olivier et al, 2018) and was probably controlled by a North-South differentiation in regional tectonics (Caravaca et al, 2018) and by concurrent global climatic changes (e.g., .…”

“…At both sites, ammonoid preservation is highly variable but is generally good, with test parts remaining on some specimens. Similar to many other ammonoid localities in Utah (Brayard et al, 2020), internal recrystallization is common, making the preparation of these fragile specimens often difficult. However, the extent of recrystallization at these sites is sometimes rather fine, allowing the observation of suture lines on a few specimens.…”

“…Systematic descriptions mainly follow the classification of Tozer (1981), and that further refined by Brayard et al (2013Brayard et al ( , 2020, Jattiot et al (2016Jattiot et al ( , 2017 and Jenks and Brayard (2018). Quantitative morphological ranges for identified species are illustrated by scatter diagrams based on classical measurements of the diameter (D), and corresponding whorl height (H), whorl width (W) and umbilical diameter (U).…”

“…Until new discoveries are made, it is therefore preferable to consider this species as characteristic only of the latest Smithian. 15 Superfamily Meekoceratoidea Waagen, 1895 Family Prionitidae Waagen, 1895 Genus Gurleyites Mathews, 1929 Type species: Gurleyites smithi Mathews, 1929 Remarks: Brayard et al (2020) provided a comprehensive revision and review of the taxonomy of Gurleyites smithi, previously assigned to the doubtful genus Arctoprionites. We follow the conclusions of this work and thus do not include the complete description for this species.…”